Carbon nanotube sensor

ABSTRACT

Carbon nanotubes are formed on projections on a substrate. A metal, such as nickel is deposited on the substrate with optional platforms, and heated to form the projections. Carbon nanotubes are formed from the projections by heating in an ethylene, methane or CO atmosphere. A heat sensor is also formed proximate the carbon nanotubes. When exposed to IR radiation, the heat sensor detects changes in temperature representative of the IR radiation. In a gas sensor, a thermally isolated area, such as a pixel is formed on a substrate with an integrated heater. A pair of conductors each have a portion adjacent a portion of the other conductor with projections formed on the adjacent portions of the conductors. Multiple carbon nanotubes are formed between the conductors from one projection to another. IV characteristics of the nanotubes are measured between the conductors in the presence of a gas to be detected.

RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.11/032,470, filed Jan. 10, 2005, which is a divisional of U.S.application Ser. No. 10/100,440, filed Mar. 18, 2002 which applicationsare incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to carbon nanotubes, and in particular toformation of sensors utilizing carbon nanotubes.

BACKGROUND OF THE INVENTION

Carbon nanotubes have been manufactured on substrates in a variety ofdifferent ways. Many methods of manufacturing carbon nanotubes involvesthe use of significant amounts of heat. This heat adversely affectssemiconductor circuitry already formed on the substrate. Such circuitryexhibits further doping migration and other changes when its thermalbudget is exhausted.

As the methods of manufacturing carbon nanotubes improves, more uses forthem are being discovered. A further problem is obtaining selectivegrowth patterns for the nanotubes to accomplish desired functions.

SUMMARY OF THE INVENTION

Carbon nanotubes are formed on projections on a substrate. A metal, suchas nickel is deposited on the substrate, and heated to form theprojections. Carbon nanotubes are formed from the projections by heatingin an ethylene atmosphere. A heat sensor is also formed proximate thecarbon nanotubes. When exposed to IR radiation, the heat sensor detectschanges in temperature representative of the IR radiation.

In one embodiment, spaced platforms are first grown on the substrate,and the projections are formed on the platforms. Single wall carbonnanotubes are then grown from the projections to obtain a desiredspacing. In further embodiments, milled SiO₂ surfaces are used to formthe nanotubes. Other surfaces may also be used as discovered.

Carbon nanotubes are used in forming a gas sensor in a furtherembodiment. A thermally isolated area, such as a pixel is formed on asubstrate with an integrated heater. A pair of conductors each have aportion adjacent a portion of the other conductor with projectionsformed on the adjacent portions of the conductors. Multiple carbonnanotubes are formed between the conductors from one projection toanother. In one embodiment, the conductors comprise multiple interleavedfingers with carbon nanotubes spanning between them.

IV characteristics of the nanotubes are measured between the conductorsin the presence of a gas to be detected. The gas absorbs into thenanotubes, changing their response to a voltage. In one embodiment, theheater is used to heat the nanotubes and drive off the gas, essentiallyresetting the nanotubes for further measurements.

In one embodiment, the nanotubes are formed by using the heater to heatthe thermally isolated pixel in an ethylene, methane or CO atmosphere.In further embodiments, the nanotubes are formed using an externalheater.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A, 1B and 1C are progressive cross section representations offormation of an IR sensor.

FIG. 2A, 2B, 2C and 2D are progressive cross section representations offormation of an alternative IR sensor.

FIG. 3 is a planar block diagram view of a self-heating sensor havingcarbon nanotubes.

FIG. 4 is a planar block diagram view of a thermally isolatedself-heating sensor having carbon nanotubes.

FIG. 5 is a flowchart showing the use of nanotubes as a gas sensor.

DETAILED DESCRIPTION OF THE INVENTION

In the following description, reference is made to the accompanyingdrawings that form a part hereof, and in which is shown by way ofillustration specific embodiments in which the invention may bepracticed. These embodiments are described in sufficient detail toenable those skilled in the art to practice the invention, and it is tobe understood that other embodiments may be utilized and thatstructural, logical and electrical changes may be made without departingfrom the scope of the present invention. The following description is,therefore, not to be taken in a limited sense, and the scope of thepresent invention is defined by the appended claims.

A process of forming a sensor is shown in FIG. 1A-C. In FIG. 1A, asubstrate 110 is formed of silicon or other suitable material, such assaphire or germanium or other substrate material which is acceptable forphotolithographic processes. A first metallic layer 120 is formed on topof substrate 110. The metallic layer 120 is nickel or cobalt in oneembodiment and is formed approximately 50 Angstrom thick in a well knownmanner. A temperature sensor 125 is formed of a material responsive totemperature changes, such as platinum, proximate to the metallic layer120. In some embodiments, it is directly beneath the metallic layer, andin others, it is closely adjacent the metallic layer 120 such that it isresponsive to temperature changes about the metallic layer. In variousembodiments, the temperature sensor 125 is formed prior to or afterformation of the metallic layer. A bolometer comprising a thermallyisolated structure on a silicon nitride or oxide bridge is utilized inyet further embodiments.

The metallic layer is heated at approximately 900 degrees Celsius forseveral minutes until projections 130 form as shown in FIG. 1B. The timeis temperature dependent, and other temperatures near or above a meltingpoint of the metallic layer cause the metallic layer to separate intosuch projections in a known manner.

Once the projections are formed, the substrate is exposed to ethylene atapproximately 700 to 800 degrees Celsius, forming carbon nanotubes 140extending from the projections. Further embodiments utilize methane orCO. The nanotubes tend to grow in an undirected manner, resembling atangle of hair upon completion. When exposed to infrared radiation (IR)150, heat is trapped about the nanotubes 140 in a manner similar to thatfound in black gold structures. The heat causes a change in temperaturethat is detected by the temperature sensor 125. In one embodiment, thetemperature sensor comprises a platinum resistance thermometer, and achange in resistance is measured corresponding to the change intemperature.

In one embodiment, a high fill factor of carbon nanotubes is utilized.The nanotubes are combined with the temperature sensor on a thermallyisolated structure to measure temperature rise of the nanotube absorbedradiation. The nanotubes and temperature sensor are part of a pixel inone embodiment. Multiple pixels are formed close together, and thenanotubes are not in electrical contact with a pixel. They are formed oneither a dielectric or isolated metal. The temperature sensor is alsonot in contact with the tubes. The temperature sensor may serve as aheater for formation of the nanotubes, or a separate heater or oven maybe utilized.

One potential advantage of using a carbon nanotube structure as an IRabsorber is that is provides high absorption, and also has a very lowmass. This combination of properties enables fast pixel response times.Initial experimental carbon nanotube structures have demonstratedabsorption rates of greater than 60% over a range of 3 to 14 um.

FIG. 2A-D are cross section representations illustrating formation of afurther sensor. In this embodiment, a substrate 210 has a first layer220 formed, followed by a second layer 230. The first layer in oneembodiment is platinum, or other layer having a higher melting pointthan the second layer 230. The second layer is nickel or cobalt, orother material one which carbon nanotubes will form. A temperaturesensor 235 is formed proximate the first and second layers.

Using common photolithographic techniques, several islands or platformsare formed as shown in FIG. 2B. Each island is comprised of the firstand second layers as earlier formed. Application of heat causes theformation of projections 260 out of the second layer material as shownin FIG. 2C. The resulting structures form a desired pattern of platformsor thin Ni islands ready for carbon nanotube growth. In one embodiment,the platforms are 1-5 micron rectangles, with a 1-5 micron spacing. Boththe size and spacing, as well as the projection density are easilymodified.

In FIG. 2D, following application of heat in an ethylene, methane or COenvironment, nanotubes 270 have formed from the projections, againforming a tangle which traps heat produced by IR radiation 280. Fourpoint temperature probes are used in one embodiment to ensure propertemperatures are maintained for nanotube deposition. By modifying thesize and spacing of the platforms, the density of the projections, andthe quantity of nanotubes formed, heat adsorption and trappingcharacteristics are modified.

In one embodiment, the nanotubes used to absorb IR radiation are on theorder of, or smaller than the dimensions of the IR radiation, 3-12 um.Thus, the spacing of the projections 260 should be on the same order ofsize, or slightly closer to account for curvature of the nanotubesforming between them.

In further embodiments, nanotubes growth is promoted on milled SiO₂surfaces or other surfaces yet to be determined. A fine photoresistpatter is formed on a SiO₂ film and then milled into regions. Thenanotubes grow where the milling of SiO₂ occurred.

FIG. 3 is a planar block diagram view of a semiconductor substrate 310having CMOS or other logic family circuitry 330 formed thereon.Conductors 340 are shown coupled to the circuitry 330 for connecting tofurther circuitry on or off the substrate. A sensor 343 is alsointegrated into the semiconductor substrate 310. The sensor 343 isformed in a manner which does not significantly adversely affect athermal budget of the circuitry 330.

Sensor 343 is formed on a thermally isolated portion 345 of thesubstrate. The thermally isolated portion is formed in one of many knownmanners. In one embodiment, it resembles an inverse pyramid with air orinsulator between it and most of the substrate. Points of contact withthe substrate, such as at the peak of the pyramid and at other portionsof the thermally isolated portion coupled by conductors to othercircuitry or external contacts.

A heater 350, such as a platinum contact is formed proximate thethermally isolated portion in manner that enables the heater to heat thethermally isolated portion as desired. Two conductors 355 and 360provide current to the heater to control the heat it produces. A pair ofcontacts 365 and 370 are formed, and extend onto the thermally isolatedportion. The contacts overlap for at least a portion of their travel onthe thermally isolated portion at overlapping portions 375 and 380. Theoverlapping portions 375 and 380 are adjacent to each other and runsubstantially parallel to each other in one embodiment. They areseparated a short distance compatible with growth of carbon nanotubes385 between them.

To form the carbon nanotubes, the heater heats the overlapping portionsof the contacts in an ethylene, methane or CO environment. In oneembodiment, projections are first formed, again using the heater toproduce desired temperatures. In a further embodiment, platforms areformed on the overlapping portions of the contacts with projectionsformed on the platforms. An electric field is applied in one embodimentto control the direction of growth of the carbon nanotubes, and toobtain point to point connection by the tubes. This produces a structureof nanotubes stretching between overlapping portions 375 and 380 withoutsignificantly adversely heating circuitry 330.

Further detail of one embodiment of sensor 343 is shown in FIG. 4. Thesensor is formed in a substrate 410. A thermally isolated region 415 isformed in the substrate by creating air gaps 420 on all sides of thethermally isolated region 415 in a known manner. A pyramid shapedopening is formed, with the thermally isolated region supported bymultiple supports 425. A heater 430 is formed on the thermally isolatedregion 415 in a circular pattern. The heater 430 comprises a platinumlayer coupled to contacts 435 and 440. The platinum layer is formed on aNi adhesion layer, and is passivated with SiN in one embodiment.Contacts 435 and 440 for heater 430 are supported by multiple supportsbridging gaps 420 from substrate 410.

Further bridges 425 provide support for a pair of conductors 445 and 450which extend onto the thermally isolated region 415. Conductor 445 andconductor 450 end in comb-like adjacent conductor structures 455 and460. Structure 455 has multiple fingers 465 intermeshing or interleavedwith opposing fingers 470 from structure 460. The conductors are formedin a common manner such as by vapor deposition. The fingers arepatterned in one embodiment using photolithographic techniques, or areformed by laser removal of the conductors to form a desired pattern.Plural carbon nanotubes 480 are formed from finger to finger by applyingheat from heater 430 in an ethylene, methane or CO environment. Anelectric field is used in some embodiments to produce a more directedgrowth of the nanotubes between the fingers. Structures other thanfinger like structures that provide good characteristics for formingcarbon nanotube bridges may also be used.

The sensor acts as a CO, O, or other gas sensor in one embodiment. IVcharacteristics between conductors 445 and 450 are measured. Thesecharacteristics change when the nanotubes 480 have absorbed CO. CO tendsto stay absorbed in the nanotubes for several minutes or more. Withoutdriving off the CO quickly, the sensor is slow to detect when CO levelshave changed. The heater 430 is used to heat the nanotubes to atemperature sufficient to drive off the CO at a faster rate, yet nothigh enough to cause further growth of the nanotubes or otherwisesignificantly adversely affect the structure of the senor or othercircuitry formed on the same substrate. When the CO is driven off, moreCO is absorbed, allowing the sensor to be reused multiple times.

The use of an integrated heater on a thermally isolated structure allowsthe use of low power to desorb the gas. In further embodiments, externalheaters are utilized.

A flowchart in FIG. 5 illustrates use of the sensor to sense CO or othergases whose effect on carbon nanotubes IV characteristics aredeterminable. The sensor is exposed to gas at 510, and IVcharacteristics are measured versus time at 520. The sensor may becontinuously exposed to gas, or the exposure may be halted at thispoint. At 530, the sensor is heated to approximately between 300 and 400degrees Celsius to drive off the gas. Exposure to the gas is thencontinued or started again at 510. This cycle is repeated as many timesas desired.

The electron affinity seems to vary depending on the gas being sensed.By mapping the IV responses of various known gases, the inventionprovides the ability 30 to detect various gases.

If the sensor is powered by a battery, the sensor need not be operateduntil needed. When needed, a cleansing heating is first applied to makesure the carbon nanotubes are not already saturated with gas to bedetected.

Conclusion

A method of forming carbon nanotubes has been described along withseveral uses of structures created with the nanotubes. The use ofprojections to control the density of the nanotubes provides the abilityto better control temperature response of a plurality of nanotubes toradiation. The use of integrated heaters provides both the ability toform nanotubes without adversely affecting circuitry on the same die orsubstrate, but also to produce sensors that utilize heat to improvetheir cycle time, such as by driving off gas more quickly. While thenanotubes were described as forming in environments containing specificgases, they may be formed in different environments without departingfrom the invention.

1. A method of sensing gas, the method comprising: measuring electricalcharacteristics of carbon nanotubes formed between conductors on athermally isolated surface in the presence of the gas; and heating thethermally isolated surface using a heater integrated with the thermallyisolated surface.
 2. The method of claim 1 wherein the heater extendsonto the thermally isolated surface.
 3. The method of claim 2 whereinthe heater comprises a platinum contact.
 4. The method of claim 1wherein the thermally isolated surface is heated to a temperaturesufficient to drive off gas adsorbed by the carbon nanotubes.
 5. Themethod of claim 4 wherein the temperature below the point at whichcarbon nanotubes are formed.
 6. The method of claim 1 wherein thethermally isolated surface is heated to between approximately 300° C. to400° C. to drive gas out of the carbon nanotubes.
 7. The method of claim1 wherein the electrical characteristics comprise IV characteristics ofthe carbon nanotubes.
 8. The method of claim 7 wherein the IVcharacteristics differ between difference gases adsorbed by the carbonnanotubes.
 9. The method of claim 1 wherein heating the thermallyisolated surface is performed in the presence of the gas to be sensed.10. A method of sensing gas, the method comprising: absorbing gas incarbon nanotubes formed between conductors on a thermally isolatedsurface in the presence of the gas; measuring electrical characteristicsof the carbon nanotubes; and heating the thermally isolated surfaceusing a heater integrated with the thermally isolated surface to drivegas out of the carbon nanotubes.
 11. The method of claim 10 whereinheating is done prior to absorbing the gas to drive off old gas from thecarbon nanotubes.
 12. The method of claim 11 wherein measuring isperformed after gas has been absorbed by the carbon nanotubes.
 13. Themethod of claim 10 wherein the thermally isolated surface is heated to atemperature sufficient to drive off gas adsorbed by the carbonnanotubes.
 14. The method of claim 13 wherein the temperature below thepoint at which carbon nanotubes are formed.
 15. The method of claim 10wherein the thermally isolated surface is heated to betweenapproximately 300° C. to 400° C. to drive gas out of the carbonnanotubes.
 16. The method of claim 10 wherein the electricalcharacteristics comprise IV characteristics of the carbon nanotubes. 17.The method of claim 16 wherein the IV characteristics differ betweendifference gases adsorbed by the carbon nanotubes.
 18. The method ofclaim 10 wherein heating the thermally isolated surface is performed inthe presence of the gas to be sensed.
 19. A method of sensing gas, themethod comprising: exposing the carbon nanotubes to a gas to be sensed;heating the thermally isolated surface using a heater integrated withthe thermally isolated surface to drive residual gas out of the carbonnanotubes; after heating, absorbing gas in carbon nanotubes formedbetween conductors on a thermally isolated surface in the presence ofthe gas; measuring IV characteristics of the carbon nanotubes toidentify the absorbed gas; and repeating heating, absorbing andmeasuring.
 20. The method of claim 19 wherein the carbon nanotubes areexposed to the gas to be sensed after they are heated.